Structure and bonding · GCSE Chemistry
Atoms, elements and compounds
GCSE Chemistry revision on atoms, elements and compounds: particle diagrams, chemical formulae, and how a mixture differs from a reaction that makes a new substance.
An element is one type of atom. A compound is two or more elements chemically joined in a fixed ratio. A mixture can be separated; a compound needs a chemical reaction to split.
The important bits
What you need to know
- 1
All substances are made of atoms. In GCSE Chemistry an atom is the smallest particle of an element that still has that element’s properties.
- 2
An element contains only one type of atom, for example O₂(g) or Fe(s). There are about 100 elements, arranged by atomic number in the periodic table.
- 3
A compound contains two or more elements chemically combined in a fixed ratio, for example H₂O(l), CO₂(g) and NaCl(s). The atoms are held by ionic or covalent bonds.
- 4
A mixture contains two or more substances that are not chemically joined. Air is a mixture of N₂(g), O₂(g), Ar(g) and CO₂(g). Composition can vary.
- 5
Mixtures are separated by physical methods: filtration, crystallisation, simple or fractional distillation, and chromatography. Compounds need a chemical reaction to break.
- 6
A chemical formula shows the types and numbers of atoms: CH₄ has one carbon and four hydrogen atoms; CaCO₃ has one calcium, one carbon and three oxygen atoms.
- 7
In a chemical reaction, atoms are rearranged. Mass is conserved in a closed system because atoms are not created or destroyed — they are only regrouped.
- 8
Particle diagrams must match the formula: O₂ is diatomic pairs, not single circles; NaCl is a giant lattice of alternating Na⁺ and Cl⁻ ions, not a molecule of “NaCl”.
Quotations worth analysing
Short evidence. Real method.
“An element is a substance made of only one type of atom.”
Use this to reject “oxygen is a compound because it is O₂”. O₂ is an element: both atoms are the same type. A compound needs different elements bonded together.
“A compound is a substance in which atoms of two or more elements are chemically combined.”
Chemically combined means a reaction has made a new substance with a fixed formula, not a stirred mixture. Water is H₂O; wet sand is a mixture.
“2H₂(g) + O₂(g) → 2H₂O(l)”
Hydrogen and oxygen are elements. Water is a compound. State symbols show gases becoming a liquid. Atom counts balance: four H and two O on each side.
Go deeper
Name the particles before you name the method
Examiners start with a particle picture and expect the correct word: atom, molecule, ion, element, compound or mixture. Circles of one colour on their own are atoms of an element. Pair those circles and you still have an element if they are the same colour, as in O₂ or N₂. Two colours joined in a repeating pattern are a compound. Two colours mixed but not joined are a mixture. Students lose marks by calling O₂ a compound because “there are two atoms”. Two atoms of the same element make a molecule of an element. Save “compound” for different elements bonded in a fixed ratio, then attach a formula such as CO₂ or MgO.
Go deeper
Separation is physical; decomposition is chemical
Ink chromatography, distilling seawater and filtering sand from salt solution do not make new substances. The dyes, water and sand were already there. Heating copper carbonate until it becomes copper oxide and carbon dioxide is a chemical change: CuCO₃(s) → CuO(s) + CO₂(g). A colour change, a gas, or a temperature change is evidence of a reaction, but the particle test is whether a new formula has appeared. If a question asks why a compound has different properties from its elements, say the bonding and the arrangement have changed. Sodium is a reactive metal and chlorine is a toxic gas; NaCl is an edible ionic lattice. That contrast is the point of the topic.
Go deeper
Formulae are counting instructions, not decorations
H₂SO₄ tells you two hydrogen, one sulfur and four oxygen atoms in each formula unit. Ca(OH)₂ means one calcium and two hydroxide groups, so two oxygen and two hydrogen. Brackets multiply everything inside. Students write H2SO4 without subscripts, or invent H₂S₂O₈ by doubling every number. Practise reading the formula aloud as a list of atoms, then check it against a balanced equation. In 2H₂O the big 2 is a multiplier for the whole molecule: four hydrogen atoms and two oxygen atoms in total. That skill feeds every later calculation in GCSE Chemistry.
See the idea in action
A student mixes iron filings with sulfur powder. The mixture can be separated with a magnet because iron is still iron. After heating, Fe(s) + S(s) → FeS(s). The product is not attracted to a magnet in the same way, and it has a new formula. Iron and sulfur were elements; iron sulfide is a compound. The reaction rearranges atoms; it does not create them. In a closed tube the total mass stays the same.
Exam technique
Turn knowledge into marks
On particle diagrams, count colours and whether they are joined. Same colour, joined or not, is still an element. Two colours joined is a compound. Two colours unjoined is a mixture. Then give a formula with state symbols if you can.
Common mistakes
Do not give these marks away
- 01
Calling O₂, N₂ or Cl₂ compounds because they are diatomic.
- 02
Saying a mixture has a chemical formula of its own, or that filtration breaks compounds.
- 03
Treating FeS as a mixture of iron and sulfur after the reaction has happened.
Which statement about O₂(g) is correct in GCSE Chemistry?
AIt is a compound because it contains two atoms
BIt is an element because both atoms are oxygen
CIt is a mixture of oxygen atoms
DIt is an ion because it has a 2 in the formula
Show the answer
It is an element because both atoms are oxygen. An element contains only one type of atom. O₂ is a molecule of the element oxygen, not a compound. Compounds need at least two different elements bonded together.
Quick questions
If this is the bit you searched
What is the difference between an element and a compound GCSE Chemistry?
An element contains only one type of atom. A compound contains two or more elements chemically joined in a fixed ratio, with a formula such as H₂O or NaCl.
Is oxygen a compound because it is O2?
No. O₂ is a diatomic molecule of the element oxygen. Both atoms are the same element, so it is not a compound.
How do you separate a mixture in GCSE Chemistry?
Use a physical method that matches the difference in properties: filtration for an insoluble solid, crystallisation for a soluble salt, distillation for liquids, chromatography for dyes.
Why is mass conserved in a chemical reaction?
Atoms are rearranged, not created or destroyed. In a closed system the total mass of reactants equals the total mass of products.